Floating Bridges - Pushing the Limits: New Adventures in Engineering - page 22

1978 by the local Vermont transportation district. This suggests that the average life of the previous
floating structures was about twenty-five years, which is near the age of the present bridge, which
presumably floated higher on the water when new.
Vermont floating bridge
Pontoon bridges have long been used by the military, in the tradition of Xerxes. The U.S. Army
built many such multispan bridges during World War II, some as long as 1,200 feet. During the war
in Bosnia in the mid-1990s, muddy conditions along a rising Sava River delayed the completion of
a pontoon bridge that was to carry peacekeeping troops into Bosnia. What bridges and roads had
survived the fighting would have failed under the weight of seventy-ton tanks and heavy guns being
moved into place, and so the construction of a floating bridge was essential to the mission. When
the weather permitted, twenty-four-foot-long, six-ton folded sections of steel and aluminum were
dropped into the river by a helicopter and pushed into place by boats. After eighty-five pontoon sec-
tions were in place, the largest floating bridge erected since World War II was ready to carry troops
and equipment across. A proof-test of sorts was conducted when an eighty-eight-ton tank-recovery
vehicle—one capable of towing disabled tanks—made the crossing. The bridge groaned and sank a
bit under the weight, but it performed its function—until it was disassembled that night.
Challenging construction projects are not limited to wartime, of course, and often they take
place under more tolerable conditions to produce structures of a more permanent kind. A large
but shallow body of water can be crossed relatively easily with a solid bridge set on firm found-
ations. Thus, the seventeen-mile-long Chesapeake Bay Bridge-Tunnel consists mainly of a series
of modest bridge spans set on piles driven into the bottom, with the bridge traffic carried into tun-
nels at two strategic points to maintain unobstructed shipping lanes. Twenty-four-mile-wide Lake
Pontchartrain, near New Orleans, is spanned by a similar low-level structure, without the tunnel
sections, as is the Seven Mile Bridge that carries traffic to Key West, Florida.
When a wide body of water is extremely deep, however, it is not always practical to construct
the number of deep foundations needed to support even a long-span bridge, and so if a fixed link is